1. Field of the Disclosure
The present innovation relates generally to artificial visual systems and more particularly in one exemplary aspect to computer apparatus and methods for implementing spatial encoding in artificial retina.
2. Description of Related Art
Various existing implementations of artificial retinal functionality aim at converting visual input (e.g., frames of pixels) to output signals of different representations, such as: spike latency, see for example, U.S. patent application Ser. No. 12/869,573, filed Aug. 26, 2010, entitled “SYSTEMS AND METHODS FOR INVARIANT PULSE LATENCY CODING”, and U.S. patent application Ser. No. 12/869,583, filed Aug. 26, 2010, entitled “INVARIANT PULSE LATENCY CODING SYSTEMS AND METHODS”; polychronous spike trains, see for example, U.S. patent application Ser. No. 13/117,048, filed May 26, 2011, entitled “APPARATUS AND METHODS FOR POLYCHRONOUS ENCODING AND MULTIPLEXING IN NEURONAL PROSTHETIC DEVICES”, each of the foregoing incorporated herein by reference in its entirety.
Artificial retinal apparatus (e.g., the apparatus described in U.S. patent application Ser. No. 13/152,119, Jun. 2, 2011, entitled “SENSORY INPUT PROCESSING APPARATUS AND METHODS”, incorporated herein by reference in its entirety) attempt to mimic particular spatial characteristics (horizontal connectivity) of natural retina cone cells, such as two-dimensional “difference-of-Gaussians” (DoG) spatial filter profile, and a difference filter in the temporal domain. In order to improve response to contrast changes and facilitate detection of edges, existing artificial retina implementations implement difference-of-Gaussians” spatial filter profile in the ganglion later (RGCs). Typically, the centers of the RGCs are arranged spatially as a two-dimensional (2-D) or a 3-dimensional (3D) structure, such as a linear array, a rectangle, square, or honeycomb pattern. The spatial extents of the RGCs, in terms of the input image pixels, may overlap with multiple neighboring RGCs.
Most existing artificial retina implementations, such as the implementation illustrated in FIG. 1, comprise predetermined connectivity pattern between the output layer (e.g., the retinal ganglion cell layer 124 in FIG. 1) and the photoreceptive layer (e.g., the cone layer 114 in FIG. 1). In the implementation of FIG. 1, in order to achieve the desired spatial response (e.g., the difference-of-Gaussians) depicted by the curve 136, each retinal ganglion cell (e.g., the cell 124_1) may be pre-wired (‘connected’) to the respective cone cells (e.g., the cone cells within the broken line rectangle in FIG. 1) using connections 122 with or without preset delays. Such implementations have substantial shortcomings, as the pre-wiring of cones to neurons to form receptive fields of the latter leads to exuberant proliferation unnecessary connections, increases processing load thereby reducing performance of the processing apparatus, increases apparatus complexity and costs, and reduces flexibility. Furthermore, while the use of prewiring produces the desired DoG RGC spatial response, temporal response of natural RGCs may be not adequately reproduced as described below with respect to FIG. 2.
The plot 200 in FIG. 2 illustrates typical amplitude response of a natural cone cell as a function of input history Δt=t0−t1, where t0 corresponds to the time of a prior input (past), and t1 corresponds to the time of current input (present). The response 200 comprises a negative value interval 202 and the positive value interval 204. It follows from configuration shown in FIG. 2 that the cones produce optimal response when a negative in-center stimulus is followed by a positive on-center stimulus.
Most implementations that employ pre-determined spatial response (e.g., DoG response 136 in FIG. 1) generate time-space separable responses, typically expressed as:H(r,t)=(r)T(t)  (Eqn. 1)While time-space separable response of, e.g., Eqn. 1 may be more straight forward to implement, such responses are suboptimal with respect to detecting moving visual stimuli. In other words, the time-space separable configuration described by Eqn. 1 and illustrated in FIG. 2, responds most optimally to a stimulus frame that simultaneously comprises positive center component and negative surround component; or comprises negative center component and positive surround component, as shown by frame pairs (242_2, 244_2) and (242_1, 244_1), respectively, in FIG. 2.
However, it is often desirable, when constructing artificial retina implementations, to reproduce time-space non-separable response of the neuroretina, such as shown for example in FIG. 12, below. The non-separable response of the neuroretina allows to, inter alia, facilitate feature recognition that is based on changes of contrast within the stimulus with time. Space-time non-separable receptive fields are often beneficial for motion detection and direction selection in visual processing.
Accordingly, there is a salient need for apparatus and methods for implementing retinal cone connectivity that does not rely solely on pre-wired connectivity and which provides improved temporal and spatial response.